Petroleum exploration activities occasionally require specialized drilling techniques to optimise production from certain types of reservoir stratum. One such drilling technique is known as xe2x80x9cunderbalancedxe2x80x9d drilling, which employs singly or a combination of nitrogen, carbon dioxide or other inert gasses, and drilling mud as the primary composite drilling fluid. In this situation, down hole pressure of the composite drilling fluid is monitored within the drill string bore and the well annulus, with the goal of preventing formation fracture due to overly high gas pressures. Another goal of underbalanced drilling is to minimise loss of the composite drilling fluid to the formation, which can be re-circulated until drilling is complete. Clearly, a specialized drilling device is needed to measure the drill string and well bore pressures to make underbalanced drilling possible.
Although there are a variety of devices for measuring downhole drilling fluid pressure, some of the devices require a temporary cessation of drilling operations, which in some cases incur cost and time delays unacceptable to drilling operators in the competitive exploration market. Such a system is described in Canadian Patent 607,352. Other types of systems allow downhole pressure measurement while drilling, generally making use of electronic pressure measurement tools rigidly fixed to the lower portion of the drill string, near the drill bit. While satisfactory for this service, such devices are irrecoverable in the event that this section of the drill string becomes stuck downhole, and consequently abandoned if efforts to free it are unsuccessful. Typically the drill string above the stuck section is disconnected in some fashion and brought to the surface, leaving behind the drill motor, drill bit, pressure measurement tools and the lower section of the drill string. Examples of such systems are described in U.S. Pat. Nos. 4,297,880 and 4,805,449, which are capable of sensing drill bore and annulus pressure, but as mentioned are irrecoverable in the event of drill string abandonment due to their mechanical design.
There is a significant need for an electronic downhole system that measures pressure in the drill string bore and the well annulus (the area between the collar OD and the well bore), that is retrievable and re-seatable, and reports pressure measurements to the surface in a timely fashion. Such a system permits drillers to make real-time decisions on how to proceed with the drilling operation based upon this and other information. The value of such a device is greatly enhanced by providing retrieval and reseating capabilities. Retrieval permits the recovery of the device in situations where the drill string becomes stuck and must be abandoned. However, certain situations arise where the tool must be recovered temporarily and then returned to the end of the drill string so that the drilling job may be continued. This is known as re-seating, and offers a level of operational flexibility not observed in the general market for similar devices.
The Dynamic Pressure Device (DPD), in accordance with an aspect of this invention measures pressure in the drill string bore and the well annulus (the area between the collar OD and the well bore) and reports the measurement to a transmitter located within the tool string. The transmitter communicates this information to the surface, where drillers make decisions on how to proceed with the drilling operation based upon this and other information.
In accordance with another aspect of the invention a drill string section for use in making up a drill string for oil and gas drilling is provided. The drill section carries instrumentation for Measurement While Drilling and Logging While Drilling operations, said instrumentation including a Dynamic Pressure Device for measuring drill string bore pressure of incoming pressurized drilling fluid, and drill string annular pressure of returned pressurized drilling fluid, said instrumentation being retrievable from said drill string when said drill string section is stuck or otherwise abandoned downhole, or otherwise reseatable into said drill string as required when drill string is deemed operational and fit for continued drilling. The drill string section comprises:
i) a length of drill string pipe having a bore defined by an inner surface of a pipe wall which has an outer surface,
ii) a cylindrical landing sleeve and a support for centering said sleeve in said pipe bore,
iii) communicating ports extending through said drill pipe wall from said outer pipe surface to said inner surface and through said support to an inside surface of said landing sleeve,
iv) said instrumentation being provided in an elongate cylindrical tool shell, spaced apart seals which engage said inside surface of said landing sleeve and the outer surface of said tool shell, means for locating said communicating ports between said spaced-apart seals,
v) said instrumentation in said tool shell having a first terminated passageway in said tool shell between said seals which communicates with a pressure sensor within said shell to sense thereby said drill string annular pressure,
vi) said instrumentation having a second terminated passageway in said tool shell in communication with said drill string bore and in communication with a pressure sensor within said shell whereby said drill string bore pressure is sensed.